Container number check digit

Every freight container carries an 11-character container number — three-letter owner code, equipment category (U freight, J detachable, Z chassis/trailer), six-digit serial, and a modulo-11 check digit.

Depot systems and documentation platforms validate the check digit before accepting a record. ConPDS Checker and Tracker run the same check at capture; this page is for manual verification or completing a partial read.

ISO 6346 uses a modulus-11 algorithm over the first 10 characters. Each letter maps to a numeric value starting at A = 10, counting upward but skipping every multiple of 11 (so there is no 11, 22, or 33 — B = 12, L = 23, U = 32, Z = 38). Digits keep their face value.

Each of the 10 values is then weighted by 2 raised to its position (2⁰ for the first character through 2⁹ for the tenth), the weighted values are summed, and the sum is reduced modulo 11. A result of 10 wraps to 0. For CSQU305438: C = 13, S = 30, Q = 28, U = 32, then 3, 0, 5, 4, 3, 8 — weighted and summed, the total mod 11 gives 3.

The doubling weights are what make the scheme useful in practice: a single mistyped character, or two adjacent characters swapped, almost always changes the computed digit — so the error is caught at entry instead of surfacing weeks later as a photo set filed under the wrong unit. See the ISO 6346 glossary entry for the owner code, equipment category identifier, and serial number that make up the rest of the container number.

Entering container numbers manually is where filing errors start. ConPDS Checker validates at capture; the email dropbox auto-links photos sent by hauliers and third parties. When charges land later, see how to avoid container repair charges.